Reduced endowment of the nephrons, the basic functional units of the kidney, is associated with future development of hypertension and even chronic kidney disease (CKD), ultimately leading to end stage renal disease (ESRD), a significant, growing economic health burden in the US. The PI has previously identified that the cap mesenchyme is a multipotent self-renewing nephron progenitor population during mammalian kidney development. It is critical to understand how cap mesenchyme cells are regulated during kidney development in order to develop therapeutic approaches to increase nephron endowment in situations like prematurity or malnutrition. Renal-coloboma syndrome (RCS) is a congenital developmental disorder characterized by renal hypodysplasia with reduced nephron numbers caused by PAX2 mutations. Although Pax2 has been widely recognized as an important factor for kidney development over the past two decades, Pax2 function in the developing kidney has not been investigated in vivo. Our central hypothesis supported by extensive preliminary data is that Pax2 function maintains the nephron lineage by repressing trans-differentiation into interstitial cell fates in the cap mesenchyme. In the Aim 1, we will determine cellular mechanisms for the trans-differentiation by molecular characterization of intermediate and terminally trans-differentiated cell states. We will also test whether Pax2 is sufficient to specify cap mesenchyme. In the Aim 2, we will distinguish Pax2 functions in the cap mesenchyme and differentiating nephron cells by comparing stage-specific Pax2 mutants before and after the onset of nephron differentiation. In the Aim 3, we will test our hypothesis that integrin ?8 activity for maintenance of the nephron lineage, acting downstream of Pax2 in cap mesenchyme cells. Results from our proposed studies will initiate to uncover the genetic networks in nephron progenitor cells regulating the lineage boundary to maintain the nephron compartment during formation of the functional kidney with the full complement of nephrons.